I feel like Kaido and Orochi should have become one character: Kurozumi Kaido.
Even before he was born, his race (Oni race) had long been a persecuted minority in Wano. Kaido's parents, though, always tried to lift his spirits. They told many stories to him, including of a weapon once kept by the Onis in Wano: Pluton. More importantly, they told him that even Onis like them deserve to have a dream and be happy.
During a power struggle between some corrupt lords in Wano for the position of Shogun, the Oni race was scapegoated after an important figure was murdered and then the Oni race was massacred by the Wanonese. Kaido's parents died protecting him, but not before sending him off to the seas. Due to his physical toughness, he was carried out of Wano until he landed far away.
This is where he started a career as a criminal and then a mercenary, until he met Xebec during a mission and was drawn to Xebec's dream as the King of the World. Xebec took him in and became a father figure for him, which reminded him of his parents. He began viewing might to be the thing that makes right, and it is only the strong that can shape the world. This is where he heard about Joyboy etc.
However, Xebec died in God's Valley and Kaido got captured by the WG despite eating the Uo Uo. Jailed in PH as a lab rat, he became increasingly disillusioned with humanity since the WG kept repeating the mantra that freaks like him is only fit for hell, not the world.
This is also where he met and befriended King. They bonded over being the last of their race. Having had his life trampled and his dream crushed many times over, Kaido began believing that the only salvation for this world is ots destruction. This is how he started seeing Joyboy, the Savior of the World, as his final obstacle. He then broke out of PH with King and they started BP.
Years after he became a Yonko, he returned to Wano with the intent of acquiring Pluton to use it to destroy the world. But Wano is now at peace under the leadership of Oden, a former member of both JRP and WBP.
Kaido then cooked up a plan ala Crocodile by having Queen create a virus that would infect most Wano citizens who aren't Oden, his family and his retainers. His chaos agent then sowed distrust among the populace and led them to falsely accuse the Shogunate to be hoarding all of the medicine (since Oden and his family + friends are unaffected).
Oden spent 5 years going around the country trying to cure his people, but got lashed out instead. Kaido, meanwhile, took his crew to Wano and started giving out antidote to anybody who joins his crew or rises against Oden. Secretly, he gave the newer recruits an actual antidote and the supporters-but-not-recruits a temporary antidote. This is how Kaido strengthened his army for 5 years.
After enough of the populace had it Oden's desperate tries to help, Kaido revealed himself to be the savior of Wano. We then got Oden vs Kaido where Kaido won fair and square.
In Oden's execution, Kaido revealed everything before killing off Oden, proclaiming that this is his revenge for Wano who made him alone in the world. Now many Wanonese have to keep returning to him if they want to keep getting the antidote, while his new recruits have burned all their bridges.
Kaido then made Onigashima his base like in the story. He made King the ruler of the Capital and nearly all of the Wano island but Udon, which was bequathed to Queen for R&D purposes. Jack functioned as their errand boy like in the story.
He started gaining more info about Pluton so he can unlock it and use it to destroy the world. He found out that Pluton is under the sea and had Jack go there, but it is buried underneath the island itself and Jack couldn't do it because his strength was not enough.
Kaido then contacted Doffy and Caesar to create Smiles, which will grant its user a partial DF power while still being able to swim for a limited amount of time. Kaido calculated that he needed at least 500 functioning smile users to give Jack a hand. His wrath against Luffy came to be because taking down Doffy and Caesar means Kaido's dream of acquiring Pluton is halted indefinitely.
This is how Beast Pirates vs Straw Hat Pirates began.
@MonsterKaido Did I do it better than Lolda?